The present invention relates to a light source used in optical communications, optical transfer technology, and optical information storage technology.
Semiconductor lasers are widely use nowadays as light sources infields such as optical communications, optical data transfer, and optical information storage, because of the coherency of the light radiated therefrom, the possibility of high-speed operation, or their extremely small size.
A semiconductor laser is mounted on a metal component such as a lead frame or metal block, for various reasons such as to ensure a thermal path way because optical output varies subtly with temperature change during the emission of light that has been stimulated by the injection of a current from an external source. However, to mitigate differences in the coefficients of thermal expansion of the metal and the semiconductor material of the semiconductor laser, the laser is first mounted on a mounting body called a xe2x80x9csubmountxe2x80x9d made of a material such as Si or AlN, before being mounted on the metal component.
A semiconductor laser comprises a resonator which has a pair of reflective mirrors and a medium with amplification ratio of at least 1 therebetween. An edge-emitting type of semiconductor laser has become more popular because cleaved facet planes of the crystal can be utilized for the reflective mirrors of the resonator and because the distance through the amplification medium can be easily increased.
On the other hand, it is possible to create a highly reflective mirror using a multi-layer structure of semiconductors and dielectric materials, or the like, and thus implement a surface-emitting laser that emits light in the normal-vector direction of the substrate. However, the technology required for this implementation is not yet sufficient and there are still technical problems to be solved. For example, the surface-emitting laser is still only at the research stage for some materials that is necessary to realize the required wavelength of emitted light. This is why most of the semiconductor laser light sources used in manufactured products are of the edge-emitting type.
When an edge-emitting semiconductor laser is mounted on a mounting body, however, problems such as those described below occur. An active region of the semiconductor laser can be used as a wave-guide structure having an extremely small cross-section, to increase the amplification efficiency and thus prevent losses due to the leakage of light from the amplification region, but this will cause diffraction of the light beam emitted from the end facet so that it expands.
In general, since it is possible to use crystal growth techniques or the like to form a thin region of wavelength order of magnitude in the direction perpendicular to the element substrate, light can be confined into a region of wavelength order of magnitude. In a direction parallel to the substrate, on the other hand, it is difficult to confine light within the wavelength order of magnitude because the confining region is formed of a planar structure, and the confining is also done within a region that is broader than the wavelength, even though that prevents any rise in the element resistance.
For that reason, the angle of diffraction diverges much more in the perpendicular orientation than in the parallel orientation. The angle of divergences of a light beam in the perpendicular orientation and the horizontal orientation become different.
For example, in the perpendicular orientation the divergence angle is on the order of 30 degrees that subtends 1/e2 of the optical intensity on the optical axis. In contrast to this, in the horizontal orientation the divergence angle is approximately 10 degrees that subtends 1/e2 of the optical intensity on the optical axis.
When a device is mounted on a flat mounting body, a part of the light beam comes into contact with the mounting surface in the vicinity of the element (such as within a distance of 200 microns when the light-emitting portion has a height of 100 microns from the mounting surface). Thus, the part of the light beam is obstructed, due to reflection, scattering and/or absorption at the mounting surface.
This will have an adverse effect during the connection of an optical fiber to an optical pickup that uses this light beam. It is therefore necessary to use some skill when mounting the device, in order to prevent kicking in the vicinity of the peripheral edge of the mounting body. This makes it necessary to limit the positional relationship between the semiconductor laser and the mounting surface, reducing the degree of freedom of installation.
One method of solving the above problems is proposed in Japanese Patent Application Laid-Open No. H05-315700. That is, a semiconductor laser element is mounted in a recess portion formed in a silicon substrate that acts as a mounting body, and a light beam that has been reflected by a wall surface of the recess portion is extracted as information.
With this configuration, the output light is reflected upward in the vicinity of the semiconductor laser element, in other words, before it diverges greatly, making it possible to extract an output beam that substantially retains its original shape, with little interference from the mounting surface and without having to consider any particular positional relationship with the mounting surface.
The optical output of a semiconductor laser varies subtly with changes in the ambient temperature. For that reason, both the semiconductor laser and the mounting substrate are mounted together on an element that enables temperature control, such as a Peltier cooler.
However, the mounting substrate and the mounting body substrate have a certain thermal capacity despite their small dimensions so, if accurate optical output control is required, the actual output beam is monitored and feed back control is imposed on the driving current circuitry. This is called automatic power control (APC).
In an edge-emitting semiconductor laser, two end facets formed at cleavage planes or the like are used as mirrors, but the output beam that is emitted is symmetrical in the forward-backward direction, provided that the reflectivity of the end facets is not particularly limited. It is possible to configure this APC by using a light-detecting element to monitor the light that is output from the rear, but this reduces the utilization efficiency of the light because the monitored light does not contribute to the signal.
For that reason, a system that requires a higher output or a higher efficiency could employ a method of increasing the utilization efficiency of the light by simply using a dielectric multi-layer film to increase the reflectivity of the end facet at the rear. In such a case, the light emission from the rear that can be used for monitoring is reduced, worsening the S/N ratio and impeding accurate APC.
This makes it necessary to monitor part of the light emitted from the forward end (the signal light), and this control method is called front-monitored APC (hereinafter abbreviated to FAPC). It should be noted, however, that if part of the beam is divided up for the purpose of FAPC, the output beam shape can change, in a similar manner to that of the obstruction of the beam described previously.
To prevent this problem, the previously mentioned Japanese Patent Application Laid-Open No. H05-315700 proposes the structure described below. The configuration, as shown in FIG. 9, is such that a mirror 925 formed on the mounting body 902 is made of a semi-transparent film and a n-type diffusion region 924 is formed behind the mirror 925 by a method such as thermal diffusion. The p-n junction that is formed around the border of the n-type diffusion region 924 acts as a photodiode element to detect the emitted light from the laser diode 901.
This structure ensures that the shape of the light beam that is reflected by the semi-transparent film 925 is emitted substantially unchanged, although the light beam has a reduced intensity. In addition, the formation of a photodiode element in the mounting body makes it unnecessary to add another light-detecting element, reducing the number of components and thus enabling an inexpensive, compact device, from the viewpoints of reducing the cost of the photodiode element and the installation costs involved with positioning it.
Furthermore, it is also possible to integrate an optical head that enables both transmission and reception, by providing another light-detecting element for reception on a flat portion. If the directions of input and output of the light are substantially the same in such a case, the configuration of the input-output system for the optical transfer path becomes simpler, making it preferable to form a surface-incident type of photodiode therewith.
However, the above described prior-art example has a further problem, as discussed below. That is, part of the light beam emitted from the semiconductor laser passes through the semi-transparent film 925 that is formed on the inclined plane and enters a photodiode element region, but part of it is first absorbed at a region (an n-type diffusion region 924 in FIG. 9), where it generates electron-hole pairs that drift approximately the distance of the diffusion length thereof then recombine and disappear.
On the other hand, electron-hole pairs excited by the light that has reached a depletion layer formed by the p-n junction, without being absorbed, travel as far as the anode and cathode, respectively, under the influence of a reverse bias applied from the outer source, to create a photo current.
For that reason, it is necessary to make the optical absorbency of the diffusion region 925 as small as possible, in order to increase the quantum efficiency of the photodiode. Control over the diffusion depth is therefore an important factor in determining the capabilities of the photodiode element. However, if the material of the mounting body substrate is highly light-absorbent at the wavelength of the semiconductor laser light, it is quite difficult to improve the quantum efficiency of the photodiode. For example, in the case of a blue semiconductor laser that oscillates at a wavelength of approximately 400 nm on a mounting body that is made of silicon, as in the prior-art example, approximately 55% of the emitted light is absorbed within a distance of only 0.1 xcexcm in the mounting body, reducing the light entering the depletion region to less than half and thus lowering the quantum efficiency.
A graph of the relative intensity of a transmitted light of wavelengths of 650 nm and 400 nm, incident on silicon, versus depth thereof is shown in FIG. 10A and a graph of the relative transmission power for light of wavelengths of 1300 nm and 780 nm, incident on InP, versus depth thereof is shown in FIG. 10B.
It is clear from these graphs that the relative intensity of the transmitted light decreases more rapidly as the wavelength becomes shorter. Thus, there is a problem in that very slight differences in diffusion depth due to a fluctuation of process factors, such as diffusion temperature of diffusion time, can have a huge effect on quantum efficiency. This is particularly true of an inclined plane that has been processed by a method such as etching, where crystal defects and/or surface roughness caused by the processing act as seeds to facilitate the occurrence of abnormal diffusion, making it more difficult to impose strict control over the diffusion depth.
The present invention relates to a semiconductor laser device wherein an inclined plane formed in a mounting body acts as a reflective mirror to emit light in a direction that is perpendicular to the mounting surface, which solves the above described problem in that the amount of light that is absorbed before it reaches a light-detecting layer (depletion layer) is strongly dependent on diffusion depth by a suitable combination of the materials of the semiconductor laser and the mounting body substrate.
The present invention solves the above described problem by forming a pin-type photodiode on an inclined plane that forms a half-mirror for a mounting body that supports a semiconductor laser element, and also ensuring that the light beam is input to a light-detecting layer made of substantially intrinsic semiconductor with low impurity (i-type layer) without passing through the impurity-diffused layer, by forming the insulation layer on outer surface of the inclined plane.
This invention provides a semiconductor laser device that comprises a mounting body having a step including an upper level, a lower level and an inclined plane provided between the upper and lower levels; and a semiconductor laser element mounted on the lower level, the mounting body further having a pin-type photodiode including a region of a first conduction type, an i-type region and a region of a second conduction type, a portion of laser beam emitted from the laser element being reflected by the inclined plane and output therefrom, a remainder of the laser beam emitted from the laser element being incident into the mounting body through the inclined plan and detected by the photodiode, the i-type region of the photodiode being provided on the inclined plane so that a major part of the reminder of the laser beam can enter the i-type region without passing the region of a first conduction type and the region of a second conduction type of the photodiode.
This invention also provides a semiconductor laser device comprising: a mounting body; and a semiconductor laser element mounted on a major plane of the mounting body, the laser element emitting a laser beam in a direction substantially parallel to the major plane, the mounting body having a pin-type photodiode including a region of a first conduction type, an i-type region and a region of a second conduction type, a major portion of the laser beam emitted from the laser element being incident to an inclined plane of the i-type region, a first part of the laser beam reflected by the inclined plane being output therefrom in a direction substantially normal to the major plane of the mounting body, a second part of the laser beam entering the i-type region through the inclined plane being detected by the pin-type photodiode.
This invention also provides a semiconductor laser device comprising: a semiconductor substrate of a first conduction type, a semiconductor laser element mounted on the substrate, an absorbing layer with low impurity provided in front of a light emitting surface of the laser element, and a semiconductor region of a second conduction type provided on the absorbing layer, the absorbing layer having a lower impurity level than the substrate and the semiconductor region, a major portion of a laser beam emitted from the light emitting surface of the laser element being incident to an inclined plane of the absorbing layer, a first part of the laser beam reflected by the inclined plane being output therefrom, a second part of the laser beam entering the absorbing layer through the inclined plane being detected by a pin-type photodiode, the photodiode including the substrate, the absorbing layer and the semiconductor region.
The present invention reduces light absorption, without contributing to the photo current, and also ensures there is no dependency on diffusion depth by a configuration that ensures that a light beam is input to a light-detecting region (i-type layer) without passing through a diffusion layer, by forming a pin-type photodiode on an inclined plane on which is formed a half-mirror film of a mounting body on which a semiconductor laser device is mounted, then forming the i-type layer on the outer surface of the inclined plane. This makes it possible to provide a semiconductor laser device with an attached FAPC monitoring photodiode that is compact, can be integrated together with another signal-receiving photodiode and has a highlight-detecting efficiency.